Integrated circuits incorporate complex electrical components formed in semiconductor material into a single circuit. Generally, an integrated circuit comprises a substrate upon which a variety of circuit components are formed and connected to form a circuit. Integrated circuits are made of semiconductor material. Semiconductor material is material that provides for the formation of junctions depending on how it is doped, and by the fact that the resistance of the semiconductor material can vary by many orders-of-magnitude, also depending on the concentration of impurities or dopants. Semiconductor material is used to make electrical devices that exploit these properties.
Common devices formed in an integrated circuit are bipolar transistors. A bipolar transistor comprises a collector, emitter and base created by junctions formed in the substrate along with other devices that make up the integrated circuit. Examples of integrated circuits incorporating bipolar transistors are subscriber line interface IC's (SLIC's) and bipolar linear voltage regulators. A known limitation of bipolar transistors is forward second breakdown (FSB). FSB occurs when a bipolar transistor is being operated at high collector current and high collector-base voltage. These combined conditions result in high power dissipation which has a tendency to develop a local hot spot. Failure will occur at the local hot spot if the temperature goes to high.
In further detail, a hot spot can develop due to the negative temperature coefficient of the base emitter voltage (Vbe) at constant collector current. Any non-uniformity (such as a fluctuation in base doping) in a portion of the device that results in a locally higher collector current will lead to higher power dissipation in that part of the device. The increased power will induce a local increase in temperature. The increased temperature will induce further increase in current due to negative temperature coefficient of Vbe. This positive feedback mechanism can raise the local temperature (called a local hot spot) high enough that the device fails.
One method for improving the FSB performance of a bipolar device is to form a resistance in series with the emitter to provide negative feedback to compensate for the positive feedback caused by the negative temperature coefficient of Vbe. This can be done by breaking the emitter into several individual emitter segments and forming a resistor in series with each segment. Another method is to use multiple resistors, each in series with several emitter segments, to reduce the number of resistors required and to simplify the connection of the resistors to the emitters. Such resistors are often referred to as ballast resistors or emitter resistors.
Emitter resistors as described above, improve FSB but still further improvements are desired. A known characteristic of a relatively large transistor device having a plurality of emitters or a simple large emitter is that the device will typically have non-uniform temperature even when the emitter current is uniformly distributed. For example, one common occurrence found in large transistors is that the temperature is highest in the center and lowest at the perimeter as a result of the design of the transistor. Another example of a common occurrence is that local hot spots develop as the result of heat generated by one or more adjacent devices formed in the same integrated circuit. This temperature non-uniformity can be particularly acute in devices made in SOI wafers because the oxide forming the isolation has much lower thermal conductivity than the silicon. As a result, large lateral thermal gradients can develop that effect the FSB. It is desired in the art to have an integrated circuit with a bipolar transistor device with improved FSB.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an integrated circuit having a bipolar transistor with improved FSB.